Silicone rubber compositions are used in a variety of applications because they form cured products having excellent properties such as weathering and heat resistance as well as favorable rubber properties such as hardness and elongation. Because of surface tack, a problem of dust deposition arises when silicone rubber compositions are used as coatings on electric and electronic parts.
Silicone varnishes have solved the problem, but are susceptible to cracks. For packages of electric and electronic parts and the like, a silicone composition capable of forming a cured product which avoids dust deposition on its surface and have satisfactory crack resistance and impact resistance is desired. There is also a demand for a silicone composition which is solid or semisolid at room temperature so that it can be cured on the existing molding machines.
As is known in the art, silicone rubber compositions of the addition cure type can be improved in cured strength by compounding resinous organopolysiloxane. The incorporation of resinous organopolysiloxane is effective in increasing the cured strength, but still leaves the problem of dust deposition because the surface remains tacky. Instead, hard resins are insufficient in impact resistance, suffering a serious problem of cracking in a thermal shock test.
Reference should be made to JP-A 2005-76003 and JP-A 2004-339482 corresponding to U.S. Pat. No. 7,019,100.